1. Field of the Invention
The present invention relates to a receiving apparatus based on Orthogonal Frequency Division Multiplexing (OFDM) in which frequencies of sub-carriers are orthogonally assigned each other in each symbol period, in particular, to an OFDM receiving apparatus and a receiving method thereof for performing diversity receiving so as to obtain a channel characteristic that allows a second or later delay wave to be cancelled or weakened.
More specifically, the present invention relates to an OFDM receiving apparatus and a receiving method thereof for performing selective diversity receiving for signals with a plurality of antenna elements for each sub-carrier so as to improve a frequency characteristic, in particular, to an OFDM receiving apparatus and a receiving method thereof for performing selectively diversity receiving based on a condition of a transmission path and in consideration of power consumption of the receiving apparatus.
2. Description of Related Art
In recent years, mobile communication apparatuses such as cellular phones and in car telephones are increasingly used and on demand. Nowadays, most people are using mobile communication apparatuses, which are increasingly being recognized as essential on their social lives. However, when wireless transmission is performed in a mobile transmission environment, the quality of transmission signals is remarkably deteriorated due to fading.
As a technology for accomplishing high-speed, high-quality wireless transmission, the so-called Orthogonal Frequency Division Multiplexing (OFDM) system has attracted attention. The OFDM system is one type of multi-carrier transmission system. Frequencies of each carrier are orthogonally assigned each other in each symbol period.
As an example of information transmission based on the OFDM system, serial information that has been transmitted is converted into parallel information in each symbol period that is lower than the information transmission rate. A plurality of parallel data is assigned to respective carriers. The parallel data of each carrier is modulated. Inverse Fast Fourier Transform is performed for the modulated data of each carrier. As a result, the data is converted into time domain signals while the orthogonality of carriers is kept in the frequency domain. The resultant time domain signals are transmitted.
For example, when data of each sub-carrier is modulated based on Binary Phase Shift Keying (BPSK) and a serial signal is converted into parallel signal in a symbol period that is 1/256 of the information transmission speed, the number of carriers is 256. As a result, the inverse FFT is performed for 256 carriers (or sub-carriers). The demodulation is performed in a reverse manner, that is, the FFT is performed for a signal in the time domain, which is converted into a signal in the frequency domain. Signals of individual carriers are demodulated based on modulating systems corresponding thereto and information of the original serial signal is reproduced.
Experimental results show that the OFDM transmission system has a satisfactory transmission characteristic in the environment in which a delay wave is present. For example, the IEEE 802.11a standard, which is well known as a wireless LAN standard, uses the OFDM system in a5 GHz band to accomplish a transmission rate of up to 54 Mbps.
When a same volume of data is transmitted, the OFDM transmission system has a longer symbol period than the single carrier transmission system. As a result, the OFDM transmission system has a characteristic in which it has a resistance against fading such as multi-path fading (in which the delay time difference between incoming waves is large) and selective fading. However, it cannot be said that transmission based on the OFDM system has a strong resistance against flat fading in which the delay time difference between incoming waves is small.
FIG. 1 shows a frequency characteristic of an OFDM signal in a multi-path environment. In a communication path in which a second delay wave (an interference wave such as a reflection wave) having an amplitude ρ and a delay τ against a first incoming wave (for example, a desired wave such as a direct wave) is received, the OFDM signal has a frequency characteristic in which a signal amplitude is (1−ρ) with every frequency difference 1/τ. In particular, when the size of an interleaver is M×N and the carrier interval is Δfc, if M/τ=Δfc or N/τ=Δfc is satisfied, the amplitudes of code symbols that have been interleaved on the reception end successively decrease. As a result, burst errors take place.
Diversity receiving which uses signals received by a plurality of antenna elements that are disposed in a manner in which correlations of signals become small is effective for signals of carriers in which the amplitude of a reception signal decreases. The diversity reception is exemplified as selective diversity and maximum ratio combining diversity. The selective diversity selectively uses a reception signal that has the strongest power in a plurality of reception signals. The maximum ratio combining diversity demodulates a plurality of reception signals and combines signals having the maximum ratios. When these diversity technologies are compared with respect to the circuit scales of apparatuses, since the selective diversity is capable of combining receiving systems into one after the reception signals have been selected. In contrast, since the maximum ratio combining diversity requires a plurality of receiving systems corresponding to the number of reception signals until the reception signals are demodulated, the scale of the apparatus becomes relatively large.
FIG. 2 shows an example of a structure of an OFDM receiving apparatus that uses an IEEE 802.11a array antenna that selectively combines (selects and combines) reception signals according to a related art of reference, for example, Yoichi Matsumoto, Nobuaki Mochizuki, Masahiro Umehira (joint authorship), “OFDM Sub-Channel Spatial Combining Transmission Diversity for use with TDMA-TDD Broad Band Mobile Wireless Communication System,” Technical Report, Rcs 97-209, The Institute of Electronics, Information and Communication Engineers, Japan.
Reception signals received by antenna elements 1-1 to 1-L are down-converted from RF frequency band signals into base band signals by RF and IF circuits 2-1 to 2-L, respectively. Thereafter, the down-converted base band signals are converted into digital signals by corresponding A/D converters 3-1 to 3-L. The digital signals in the time domain are Fourier-transformed by digital Fourier transforming sections (DFTs) 4-1 to 4-L and the converted signals are extracted as signals of individual carriers in the frequency domain.
A selectively combining section 5 compares powers of signals received by receiving systems (each in which is composed of the antenna 1, the RF and IF circuit 2, the A/D converter 3, and the DFT 4) for each sub-carrier. The selectively combining section 5 selects a signal having the maximum power for each sub-carrier. The selected carrier is deinterleaved by a deinterleaver 6. The deinterleaved signal is decoded to original transmission information by a decoder 7.
FIG. 3 illustrates principles by which the OFDM receiving apparatus shown in FIG. 2 selectively combines signals for each sub-carrier. In the following description, for simplicity, in FIG. 3, it is assumed that the number of antenna elements of the array antenna (namely, the number of receiving systems) is two.
FIG. 3 shows powers of carriers of signals received by the antenna elements 1-1 and 1-2. With respect to reception sub-carriers at frequencies f1, f4, and f5, the powers of each sub-carrier received by the antenna element 1-1 are larger than the powers of sub-carriers received by the antenna element 1-2. In contrast, with respect to reception sub-carriers at frequencies f2 and f4, the powers of sub-carriers received by the antenna element 1-2 are larger than the powers of sub-carriers received by the antenna element 1-1.
In such case, with respect to the frequencies f1, f4, and f5, the selectively combining section 5 selects the sub-carriers received by the antenna 1-1, whereas with respect to the frequencies f2 and f4, the selectively combining section 5 selects the sub-carriers received by the antenna 1-2.
When the powers of signals received by the antenna elements of the array antenna are compared, selectively combined for each carrier, the SN ratios (Signal to Noise ratios) for each carrier may be improved, thus, satisfactory receiving performance may be achieved.
However, in the structure of the diversity OFDM receiving apparatus that selects sub-carriers as shown in FIG. 2, it is necessary to extract sub-carriers for each antenna element. As a result, each of the receiving systems has to be provided with an A/D converter and a DFT and they have to be driven as shown in FIG. 2. As a result, the circuit scale of the apparatus becomes large. In addition, when the demodulation and the DFT are operated in all the receiving systems, the power consumption of the entire receiving apparatus becomes considerably large.
In addition, it would not be necessary to selectively combine signals for each sub-carrier in a relatively satisfactory communication environment having a low error rate rather than a multi-path environment in a bad transmission characteristic. As a result, it would be redundant to operate all the receiving systems.